The organization of synapsis during integrative recombination of bacteriophage lambda has been probed by measurement of the change in supercoiling that accompanies integration. A unique loss of two superhelical turns is found, indicating the two recombining DNA double helices are brought together during synapsis in a tight, ordered structure. The role of topoismerases in recombination has been confirmed by finding specific topoisomerase cleavage produced by a purified recombination protein, Int, at the crossover locus. Although Int protein normally requires a host protein to promote integration, a variant Int protein has been characterized that accomplishes recombination by itself.